High strength low alloy steel possessing sufficient weldability containing small amounts of nb,ti,and b

ABSTRACT

HIGH-STRENGTH LOW-ALLOY STEEL KEEPING YIELD STRENGTH OF MORE THAN 70 KG./MM.2 IN A FURTHER COORDINATED RELATIONSHIP WITH SPECIFIC SMALL AMOUNTS OF ALLOYING INGREDIENTS, PARTICULARLY NB+TI+B, AND POSSESSING GOOD TOUGHNESS AND SUFFICIENT WELDABILITY THROUGH AN ORDINARY QUENCHTEMPER HEAT-TREATMENT.

United States Patent "ice 3,592,633 HIGH STRENGTH LOW ALLOY STEEL POS-SESSING SUFFICIENT WELDABILITY CON- TAINING SMALL AMOUNTS 0]? Nb, Ti,AND B Tatsumi Osuka, Junichi Tanaka, Michio Hayashida, and Yoshiaki Ono,Kawasaki-shi, Japan, assignors to Nippon Kokan Kabushiki Kaisha NoDrawing. Filed Jan. 22, 1969, Ser. No. 802,715 Claims priority,application Japan, Jan. 22, 1968, 43/3,389 Int. Cl. C22c 37/10, 39/02US. Cl. 75-124 4 Claims ABSTRACT OF THE DISCLOSURE High-strengthlow-alloy steel keeping yield strength of more than 70 kgjmm. in afurther coordinated relationship with specific small amounts of alloyingingredients, particularly Nb+Ti+B, and possessing good toughness andsufficient weldability through an ordinary quenchtemper heat-treatment.

The present invention relates to high strength low alloy steels and,more particularly, to high strength steels for welding structuralservices.

It is well-known that many high-strength low alloy steels having yieldpoints of more than 70 kg./mm. are employed widely or welding structuralservices. In such cases, both high strength and superior toughness aregenerally required. These characteristics were obtained by adding aconsiderable amount of alloying elements with ordinary quench-temperheat treatment. Such conditions of alloying elements, however, makehardenability of steels increase and are apt to bring about cracks atwelding zone of steels. For elimination of said defects, preheating ofmore than 100 C. is generally required. The higher the pre-heatingtemperatures, the higher the cost becomes. Difliculties of suchtemperature control increase also in the field of welding steels.

It has now been discovered that by maintaining specified and coordinatedranges of alloying elements, the above defects can be completelyeliminated. According to the present invention, the low carbon steelpossesses a yield point of more than 70 kg./mrn. and superior toughnessand has been proved in reduced hardenability in welding and decreasedcrack sensitivity.

It is an object of the present invention to provide a novel highstrength low alloy steel, particularly containing 0.005% to 0.04% Nb,0.0005% to 0.005 B and 0.005% to 0.05% Ti.

Other objects and advantages will become apparent from the followingdescriptions and illustrative examples. The invention also contemplatesproviding a unique steel particularly adapted to be employed in theWelding structure.

Generally speaking, the present invention contemplates the manufactureof steels heat-treated with quenching of adapting liquids (containingwater), gases (containing air), or mixtures of above media at atemperature from AC point to 1050 C. and then with tempering at atemperature of less than AC point.

The steels of the present invention are composed of the followingconstituents:

3,592,633 Patented July 13, 1971 Percent by weight C 0.05-0.15 Si0.10-0.70 Mn+Ni 0.50-2.00 But Mn 0.50-1.60 Ni 0.03-0.80 Mo 0.15-0.70 Cu0.05-0.80 Cr 0.03-1.00 Sol. Al 0.01-0.1 N 0.002-0007 Nb 0.005-0.04 B00005-0005 Ti 0005-005 The remainder: Fe and unavoidable impurities.

The reasons for the limited ranges above mentioned for the constituentsof the present invention steel are as follows:

Carbon is one of the more potent and more economical strengtheningelements. It is well known that carbon has the strongest influence uponhardenability of weld zone. Accordingly, it is to be desired that carboncontent be low as much as possible. Carbon of more than 0.05%, however,is practical from this point of steel making. The upper limit of carboncontent, which is 0.15%, is recommended, owing to the need of making0.01 to 0.03% Nb solute at a quenching temperature of 900 C. to 1000 C.,so as to cause Nb-carbide after tempering treatment.

Niobium content must be determined in consideration of carbon content.According to our experimentations, it is shown that Nb of more than0.05% scarcely contributed to strength steel. Also, it is impossible toobtain the required strength when Nb content is less than 0.005% Thesuitable Nb content in the present invention consists in the range of0.005 to 0.05%.

Manganese of 0.50% which is the lower limit is adopted in considerationof this point of steel making and of hotworkability. Also, 1.60% Mnwhich is the upper limit is determined in consideration of weldability.

Up to 0.8% nickel causes toughness of steel to increase and results ineconomical benefits.

When copper content is above 0.5%, nickel must be added simultaneously.More than 0.03% chromium possibly causes strength and hardenability ofsteel to increase.

0.15% to 0.7% molybdenum possibly causes strength of steel to markedlyincrease. However, when the content is more than 0.7%, weldability ofsteel is spoiled and when the content is less than 0.15 strength ofsteel is harmed.

More than 0.1% aluminum decreases cleanliness and spoils weldability ofsteel. Also less than 0.01% aluminum brings about a bad influence toaddition of boron in steel making stage.

When silicon content is above 0.7% toughness and weldability of steel isharmed.

Up to 0.7% boron possibly causes hardenability of steel to increasewithout a bad influence to toughness and workability of steel.

0.005% to 0.05% titanium is a useful element for improving hardenabilityof steel and for increasing strength of steel owing to precipitation ofTi-carbide in the tempering treatment stage.

Nitrogen content has a serious influence on combination of Nb-Ti-B whichis the main point in the present invention. By reasons of which nitrogenhas powerful aflinity to the above three elements, particularly Ti, thecontent must be limited within the range in which it is impossible toform its nitride. More than 0.007% nitrogen harms toughness of steel,owing to the formed Ti-nitride and causes Ti-carhide, which is effectivefor strength of steels 4 Examining the above-mentioned values, it isunderstood that maximum hardness in taper hardness test is far lowerthan the maximum value required by the same kind of high strength steel,such as Hv$430 in Japanese WES Standard. This fact shows that hardnessof welding zone to decrease. Nitrogen content of the lower limit is0.002% is markedly lowered according to keeping yield strength accordingto a restriction of steel making process. of 70 kg.mm. by means of asuitable combination of II! the range of the above-mentioned C l CNb+Ti+B and at the same time main alloying elements, position, thefollowing range is recommended as preferred such as C, Mn, Ni, Cr, Moand the like, are far less than values: ordinary high-strength steels.

Pfifcent y Weight As for crack sensitivity, also it is shown thatoccurence C 04064112 f some Crack in the present invention steels can becomi pletely eliminated on condition of simple pre-heating of MII+NI1130-0-60 75 C. in comparison with high preheating of 100 C. for Cu02043-40 ordinary high-strength steel. Cr 0420-0-60 Although the presentinvention has been described in Mo 02043-60 conjunction 'with preferredexamples, it is to be under- Sol A1 0010-01150 stood that modificationsand variations may be resorted to Nb 013054103 without departing fromthe spirit and scope of the inven- Ti (1014103 tion as those skilled inthe art will readily understand. B What we claim is: N less thanO-OOS 1. High strength low alloy steels composed of 0.05 to As mentionedabove, the present invention steel is com- 015% to t0 L posed ofalloying of a small quantity of components, and to 150% Mn, and t0 5 t080% demonstrates yield strength of more than 70 kg./mm. Cu, to 100% to(110% 0-002 to superior toughness and excellent weldability according toN, 0-005 to 0.04% Nb, 0.0005 to 0-00 heat treatment of quenching mothersteel from more than 0-005 to (105% T1 and havlhg a V 4 Strength Q more4C temperature and then of tempering at more than than Q' and POSSeSSIhgsufficleht weldahlllty- 5 C. High strength low alloy steels inaccordance with The following Table 1 shows examples of the presentinclalm 1h Whlch the alloying ingredients wmprise vention steel. InTable 1 from A to G, steels are the presto 012% c, to 050% Si, L00 t0ent invention steels. And from H to K, which are out of 020 to 040% Cu,to 050% 0420 to 0360% the range of components based on the presentinvention, 04010 to 0'050% 0.005 to (105% 0- to these steels are shownin comparison with the present in- 0.03 Ti, 0.001 to 0.003% B, up to0.005% N, vention steel. L steel is an ordinary high-strength low alloy3. High strength low alloy steel in accordance with steel. claim 1,wherein the steel following heat treatment is TABLE I Mechanicalproperties Chemical composition Chemical compound Elonga- Yield Tensiletion, vIrs, vTr,1s 0 Si Mn P 5 Cu Ni Cr Mo Nb Tl B Al N point strengthvEo 0. 0. 14....000 0.13 1.05 0.020 0.015 0.30 0.25 0.44 0.42 0.012 0.030.002 0.035 0.0051 77.1 80.0 23.1 18.7 -94 -100 13.... 0.11 0.10 1.100.018 0.011 0.31 0.20 0.40 0.42 0.015 0.03 0.002 0.030 0.0045 75.8 80.222.5 24.3 -58 -03 0.... 0.10 0.10 1.00 0.015 0.014 0.24 0.31 0.44 0.420.017 0.02 0.001 0.030 0.0035 81.2 83.4 23.3 17.9 -52 -97 D --0. 10 0.211.24 0.014 0.014 0.24 0.27 0. 0.42 0.021 0.02 0.003 0.048 0.0041 83.385.1 23.3 10.7 -00 -102 0.10 0.22 1.29 0.023 0.013 0.30 0.03 0.54 0.300.010 0.02 0.002 0.042 0.0007 78.7 83.2 24.8 18.9 -120 0.10 0.20 1.230.022 0.007 0.27 0.03 0.57 0.38 0.015 0.01 0.002 0.042 0.0005 78.7 83.123.0 19.2 -74 -111 G-.. 0.12 0.17 1.24 0.022 0.010 0.30 0.03 0.53 0.390.033 0.02 0.002 0.002 0.0002 70.0 83.1 23.9 14.9 -42 -73 H 10 0.13 1.100.020 0.010 0.30 0.27 0.44 0.40 0.012 0.03 0.002 0.040 0.0112 70.7 75.823.0 10.9 -7 -45 1--.. 0.10 0.24 1.27 0.021 0.017 0.28 0.03 0.57 0.39'Ir. 0.02 0.002 0.050 0.0004 05.1 72.0 28.0 21.5 -103 -120 0.10 0.201.20 0.021 0.013 0.28 0.04 0.53 0.39 0.010 0.00 0.002 0.030 0.0009 77.782.9 24.5 11.0 -18 -58 K... 0.12 0.23 1.30 0.023 0.015 0.38 0.20 0.480.40 0.015 Tr. 0.002 0.028 0.0110 00.3 75.2 20.7 23.9 -72 L 0.12 0.200.80 0.010 0.018 0.27 0.85 0.53 0.54 V0.05 Tr. 0.004 0.028 0.0005 77.183.0 23.5 11.3 -89 -104 l KgJmmJ. 2 KgrnJcmfi.

In Table II, the values of which hardness and crack sensitivity of B, C,D and L steels in Table I are tested by means of both taper hardnesstest and y-type slit crack test methods are shown.

quenched at a temperature from AC to 1050 C. and then is tempered atless than AC temperature.

4. High strength low alloy steel in accordance with claim 2, wherein thesteel following heat treatment is 5 quenched at a temperature from AC to1050 C. and 3,368,887 2/1968 Clark 75-128R then is tempered at less thanAC temperature.

HYLAND BIZOT, Primary Examiner References Cited UNITED STATES PATENTS 5CL 2,750,233 6/1956 Loveless 75-124 75125128R;148-31 3,216,823 11/1965Gulya 7s 124 CEL 33 a g g; 9

Patent No. 3'592'633 Dated y 1971 Inventoflg) TATSUMI OSUKA et al It iscertified t hat error appears in th e above-identified patent and thatsaid Letters Patent are hereby corrected as shown below: f

Column 4, line 24 (Claim 1, line 3) before "0.05" insert "0 15 to O 70%M0" Signed and sealed this 15th day of February 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOT'ISCHALK Attesting Officer Commissionerof Patents

